There is a possibly apocryphal story about a speaker designer showing a prototype of a new speaker to his boss. "What's the lower limit of response?" asked the executive. The instant reply: "What do you want it to be?"
The story points up the fact that frequency response charts (and therefore published response numbers) are largely arbitrary, or more correctly that they are set so as to serve the needs of the marketing department. I am sorry to say that users -- and this includes professional sound people who should know better -- are accomplices in this deception. Because they glance at loudspeaker specification sheets first and foremost to verify the low-frequency response limit, manufacturers give them what they are searching for. If they do not, their competitors will be only too happy to take over.
The published figures give the impression that nearly all speakers have the same frequency response: down to 40 Hz for reasonably-sized speakers, down to 25 or 30 Hz for very large speakers. The matter is delicate in the case of larger speakers, to be sure. A potential purchaser may be pleasantly surprised to see that a "bookshelf" speaker has response down to 40 Hz, but would almost certainly reject a large and expensive speaker with claimed response down to only 65 Hz. Yet the second claim may well be the more honest one.
Perhaps I should reword that to say that it may be the more useful one. It is rare for the manufacturer of an audiophile recording (or a professional studio monitor) to actually lie. The confusing aspect is that, under some carefully-chosen circumstances, nearly all published figures can be justified.
What figures, then, are useful? I recall that some years ago a prominent audio publication spoke in its reviews of "usable response," a term which at first glance seemed like virtually a synonym of usefulness, though in fact as nearly as I can make out it actually meant something quite different, and perhaps even its exact opposite. The reviewer might say in his review that the response was 3 dB down (i.e. half the average level) at 65 Hz, let us say, but had "usable response" down to 28 Hz. Usually this meant that sound could still be heard emerging from the speaker with a 28 Hz signal, even though the measuring instruments were oblivious to it.
Now, fairness dictates that I mention that my friends at UHF are sometimes guilty of employing the same concept. For example, the review of the Castle Inversion 15 (UHF No. 57, page 48) includes a response graph that clearly shows a sharp drop in response below 100 Hz. However, the text of the review says that "what it produces is clean until about 42 Hz." Look back at the graph, and you'll see that the 42 Hz signal, as measured by UHF's instruments was nearly 20 dB below the 1 kHz response. How "usable" is this really?
You should not, however, assume that this speaker has poor low-end performance (the text of the review certainly suggests otherwise). The manufacturer offers much more attractive figures for the Inversion 15: a stated range of 50 Hz to 20 kHz (no decibels stated). I cannot say that they are wrong to do so, only that they are clearly working from different criteria. It may in fact be argued that there is no single way to evaluate frequency response so that it is meaningful. Let us look at the possibilities, and also at the problems.
Let me take as an example the response graph of that same Castle loudspeaker, as published in the magazine. It looks like this:

Seen in this fashion it is not a graph the manufacturer would wish to include in a four-color brochure. However, note the graduations on the vertical axis, with its 5 dB steps. Let us replot the graph with 10 dB steps, as is somewhat more usual, and it now looks like this:

It does seem more flattering, with the part of the curve below 100 Hz no longer lying on the "floor" of the graph. The curve is now almost publishable.
We can massage it one more time, however. On a frequent basis, I see response graphs with 20 dB steps, at which point the variations become frankly trivial, and the speaker seems to have flat response. Even the "usable response" now seems quite reasonable.

We can make one final set of adjustment. Unlike most hi-fi speakers, the Castle has no peaks anywhere in its frequency range. The 1 kHz response is the highest point in the curve, matched by the 7.5 kHz point. The lack of peaks is actually a good feature, to be sure, but it affords us an opportunity to "pretty up" the curve by raising it slightly. And at the same time we can disguise what we have done by the simple expedient of getting rid of the decibel graduations on the vertical axis, and also eliminating some of the now superfluous horizontal graph points. The low end of the curve is now extended to only 50 Hz. The final curve is one which would warm the heart of any marketing manager:

The speaker now appears to have virtually flat response. To be fair, Castle does not publish curves, a wise policy, in my view.
Now, I have carried on this exercise using a test curve from a magazine which is known (and often criticized) for test methods that do not show up speakers at their best. Notably, the level used is rather high: the microphone is placed one meter in front of the test speaker, in line with the tweeter, and the volume adjusted so that a 1 kHz tone (or, in actual fact, a third-of-octave warble tone centred at 1 kHz) has a sound pressure level (SPL) of 100 dB at the microphone position. Why should it be so high? Of course, the resulting curve will give a good idea of the speaker's performance at high listening levels, and this is no doubt useful, but the curve would almost certainly be "prettier" at an SPL of 90 dB, a level only one tenth as loud. Many professional monitors come with curves for several reference SPL's, but for hi-fi speakers a reference SPL of 90 dB is quite common.
You may wonder why the frequency response figure at 90 dB and 100 dB should be different, since of course in a perfect speaker they surely ought to be identical. It is clear that we are some distance from perfection, however, and at lower frequencies that perfection becomes all the more elusive. Let us see why.

(What you have just read is about one third of the full article. You can read it all by ordering the print version of UHF No. 60)